YAML is whitespace-sensitive, which makes tabs and stray trailing spaces a common, hard-to-spot source of parsing errors, most YAML parsers reject tabs used for indentation outright. This tool does exactly two things, on purpose: it converts every tab character into 2 spaces, and it removes trailing whitespace from the end of each line. It deliberately does not re-indent your structure, reorder keys, or validate the document, so what you get back is your original YAML with just its whitespace hygiene cleaned up.
Because YAML's indentation carries structural meaning, safely re-indenting it requires actually parsing the document into a tree first, get that wrong and you can silently change what the file means. This tool intentionally stays simple and only touches whitespace characters, so it can never alter your document's structure or values.
The YAML specification disallows tab characters for indentation, so a file with tabs will often fail to parse in strict YAML implementations even though it might look fine in an editor. Converting tabs to spaces up front avoids that entire class of error.
On its own it's usually harmless, but it's invisible in most editors, bloats diffs when a file is checked into version control, and can occasionally interact badly with block scalars. Stripping it is a cheap, safe cleanup step with no downside.